The present invention relates to hydraulic power-assisted steering gear for vehicles, and particularly to an automatic wheel return system for use with a power-assisted steering gear.
In order to restrict the steering forces at the steering wheel, the steering gear is equipped with hydraulic power assistance particularly in the case of heavy vehicles. The deflected wheels are automatically returned to the straight position, at least when there is sufficient travelling speed, by appropriate axle geometry or by reverse rotation of the steering wheel. Under any travelling situation and even when the vehicle is at rest, the wheels can be returned automatically into the straight position by hydraulic means using a return device controlling the servo-cylinder in accordance with the existing steering deflection, without appropriate axle geometry or manual steering being necessary. An appropriate return device is described in German Unexamined Published Patent Application (DE-OS) No. 3,218,844. The threaded spindle of the steering gear is rotationally restrained in the steering geer housing by means of a cross recess coupling but it can execute small control displacements axially relative to the steering column. By this means, the threaded spindle can, on the one hand, transmit steering torques to the drop arm and, on the other, act as an axially movable control valve spool for controlling the flow of pressure medium to the servo-cylinder. During steering operations, the threaded spindle is displaced axially by a control displacement against the force of centering springs and carries, connected to it, a valve bushing which moves in a control housing. The centering springs are supported on a collar which is guided with an axial helical movement in a thread on the threaded spindle but which is supported on the steering gear housing so that it cannot rotate. After the conclusion of the steering movement and after the steering wheel has been released, the preloaded centering springs place the threaded spindle and the valve bushing beyond the neutral position in a control position corresponding to reversal of the steering gear so that the previously unloaded pressure space of the servo-cylinder now receives pressure. The return movement is concluded when the axial displacement of the collar, which depends on the amount of steering deflection, has again disappeared and the valve bushing finally takes up its neutral position. The mechanical springs of the type used are, as is known, linear work storage devices so that the force acting on the control spool can, in turn, change only linearly. This force is also felt, either diminished or magnified, as a return force at the steering wheel changing linearly with the steering deflection. Steering wheel forces increasing linearly with the steering deflection, however, do not provide steering with a satisfactory feel. The return force at the wheels also alters linearly with the steering deflection because the displacement path of the threaded spindle, which acts simultaneously as the control spool, determines the pressure to which the servo-cylinder piston is subjected. For good centering of the wheels in the straight position, however, it is desirable that a high return torque should act, particularly at small deflections of the wheels from the straight position, and that this return torque should not increase linearly with increasing steering deflection. A further disadvantage is that the way in which the force varies with deflection angle is fixed on installation of the springs and can only be changed by substantially dismantling the steering gear. Another disadvantage is that hysteresis effects can arise due to the mechanical mode of action of the return device and this makes exact setting of an unambiguous steering position for travelling in a straight line impossible. Furthermore, the special mode of operation of the steering gear (since axial movement of the threaded spindle are necessary) does not permit the working spaces of the servo-cylinder to be located around the threaded spindle in a space-saving manner.
The return device described in German Unexamined Published Patent Application (DE-OS) No. 2,324,347 for a rack steering gear again agrees in principle with the mode of operation of the return device described above. A centering spring preloaded to suit the steering gear and the steering deflection is again used, after the steering wheel has been released, to displace the control valve spool of the steering gear into a control position corresponding to a return movement into the straight position of the wheels in order, by this means, to reverse the way in which the working spaces of the servo-cylinder are subjected to pressure. The preloading of the centering spring necessary for this purpose is achieved by means of a threaded section driven by the steering column; on this threaded section, there is a nut which is axially supported by springs on a casing which is located parallel to the control spool and is solidly connected to it. The same disadvantages apply as those already enumerated for the previously described return device.
Another return device known from German Unexamined Published Patent Application (DE-OS) No. 2,331,566 is also used on a rack steering gear. Between a control spool, which can be deflected from its neutral position as a function of the steering direction, and its housing, there is an additional axially movable casing spool whose axial position is controlled as a function of the position of the rack. If there is a deflection due to actuation of the steering wheel, the control spool is displaced through a control distance against the force of a centering spring in the corresponding direction and corresponding passages are opened so that the working spaces of the servo-cylinder can be subjected to pressure. When the rack is displaced, the casing spool is also driven along into a corresponding position which does not, however, disturb the flow of pressure medium which has been initiated. After the conclusion of the steering wheel rotation, the control spool returns to its neutral central position while the casing spool is retained in its position, for example by a frictional connection. In this position of the casing spool and the control spool, passages are now open and by means of these, the servo-cylinder can be switched over. The casing spool resumes its neutral position only when the wheels are in the straight position. The adjustment of the casing spool takes place, in each case, by means of a mechanical setting device, which can also be controlled hydraulically. A disadvantage is that expensive and accurately fitting precision parts are necessary so that the moving parts can slide over one another with accurate fits and be positioned in the correct axial positions. A mechanism of this type, which is expensive, complicated and subject to wear in use, is of course subject to damage. The failure of the setting mechanism of the spool can lead to dangerous driving situations because, in the case of unfavorable spool positions, the wheels may be displaced into an unexpected position.
From German Unexamined Published Patent Application (DE-OS) No. 3,220,922, it appears as known that a control spool of a power-assisted steering gear operates with each of its end faces in a reaction chamber of the valve housing and that the reaction chambers are, on the one hand, connected via a constant throttle to a short-circuit connection. The reaction chambers are, on the other hand, each connected via a non-return valve, opening in the direction of the reaction pressure chamber, to one operating chamber each of the servo-cylinder. A pressure line leads away from the short-circuit line via an adjustable throttle to a reservoir. By altering the throttle cross-section of the adjustment throttle as a function of the travelling speed, a differential pressure corresponding to the differing pressures in the operating chambers builds up across the control spool during steering movements and this differential pressure can be felt as a manual force at the steering wheel. In the case of this hydraulic circuit, however, it is not the intention and it is also not possible to effect an automatic return of the wheels from a deflected position to the straight position because, after the control spool has returned to its neutral position (which is assumed for every constant travelling condition even if the wheels are deflected), the pressure in the operating chambers of the servo-cylinder and the reaction pressure chambers are balanced. However, automatic displacement of the control spool from the neutral position into a control position corresponding to a return steering position is necessary to provide automatic return movement of the wheels. In order to produce manual forces at the steering wheel, a large number of possible devices for producing an axial force at the control spool are, in addition, known from the literature. However, the automatic return of the wheels requires fundamentally different solutions.
One object of the present invention is to provide a simple and robust device of small installation dimensions for the automatic hydraulic return of vehicle wheels into the straight position.
Another object of the present invention is to generate manual steering forces and wheel return forces that vary non-linearly as a function of the steering deflection angle with the control spool of a power-assist valve system being subjected to corresponding forces. In addition, the force variation should also be alterable by simple means.
According to the present invention, a hydraulic power-assisted steering gear is provided for use with a vehicle having steerable wheels. The steering gear includes means for turning the steerable wheels between a straight position and at least one deflected position, manual steering means for operating the turning means in response to an instruction from a vehicle operator, power-assisted steering means for assisting the manual steering means in operation of the turning means, and return means for returning automatically the steerable wheels from a deflected position to the straight position.
The power-assisted steering means include hydraulic means for operating the turning means using a flow of pressurized working medium, a valve housing in fluid communication with the hydraulic means, and a control valve spool slidably received in the valve housing. The control valve spool is movable in the valve housing between a straight wheel position, a first steering direction position, and an opposite second steering direction position. In preferred embodiments of the present invention, the first steering direction position corresponds to a right-handed turn of the vehicle and the opposite second steering direction position corresponds to a left-handed turn of the vehicle. The turning means is operated by the manual steering means with assistance from the power-assisted steering means when the control valve spool is in either of the first or second steering direction positions.
In preferred embodiments of the present invention, the return means includes means for sensing a selected deflected position of the steerable wheels and means for continuously urging the control valve spool toward the other of the first and second steering direction positions when the steerable wheels are turned to a deflected position due to operation of the turning means so that the steerable wheels are returned to a straight position. Desirably, the selected deflected position corresponds to one of the first and second steering direction positions of the control valve spool. In preferred embodiments of the present invention, during operation of the return means the vehicle operator senses a magnified or diminished manual force indicative of the tendency of the turning means to return to its straight position via the manual steering means.
The control valve spool must be deflected from its neutral position in order to cause the operating chambers of a servo-cylinder housing the turning means to be subjected to different pressures for a steering maneuver. This is accomplished by subjecting the control spool to an external force. In the case where the deflected wheels have to return automatically (i.e. without steering support from the steering wheel even with the vehicle at rest) into the straight position, the control spool must be displaced automatically into a control position (i.c. first or second steering direction position) away from the neutral position (i.e. straight wheel position), this control position being in the opposite direction to the control position which was necessary to bring about the steering deflection. For this purpose, the control spool is continually subjected to pressure on its end surfaces while the wheels are deflected, the pressure action producing an axial force. If a corresponding force at the steering wheel does not act against this axial force, the operating chambers of the servo-cylinder are subjected to pressure corresponding to a return movement of the wheels and the wheels are hydraulically returned to the straight position.
A change to the pressure difference acting on the control spool also causes a change to the manual steering forces felt and, in addition, an alteration to the return torque at the wheels because the operating chambers of the servo-cylinder are subjected to pressure as a function of the control position of the control spool. The axial force derived from the pressure difference at the control spool acts against preloaded springs centering the control spool in a central position. In addition, the manual steering forces and wheel return torques can be easily changed by, for example, influencing the pressure level of the pressure medium by means of throttling.
The pressure difference at the control spool is controlled as a function of the steering angle by using a linear displacement/pressure or an angular displacement/pressure converter as the control element which senses the linear or angular displacement at the control linkage or steering column. The converter can, for example, interact with a pilot valve which, acting as an adjustable throttle, controls the flow of pressure medium to and from the spaces subjected to pressure at the end faces of the control valve spool.
The high degree of operational safety of the return device is advantageous, due to the use of simple constructional elements. In addition, manufacture of steering devices with a return device is possible at a favorable cost. Furthermore, these devices have practically the same external dimensions as previous steering devices without a return device. The fact that the force on the control spool, and also the resulting forces at the steering and road wheels, can be adjusted to give any variation with steering angle is also advantageous. By this means, for example, it is possible to provide relatively high return forces even at small steering angle deflections from the straight position and to maintain a curve shape which flattens with increasingly large steering angles. This makes particularly exact guidance of the wheels in the straight position possible. Such a return device can also be used in an advantageous manner in the steering devices of track guided vehicles in order, particularly in sharp curves, to preload the wheels in the direction of a track guidance flange at the outside of the curve, along which flange the vehicle has to be guided through the curve on one side.
Further objects, features, and advantages of the present invention will become more apparent from the following description when taken with the accompanying drawings which show, for purpose of illustration only, an embodiment in accordance with the present invention.